1. Field of the Invention
The present invention relates to high frequency semiconductor devices, and in particular, to monolithic microwave integrated circuit (MMIC) for use in gigahertz frequency or higher.
2. Description of the Related Art
In MMICs in which high speed semiconductors such as high-electron-mobility transistors (HEMTs) and hetero-bipolar transistors (HBTs) are integrated with passive elements such as capacitors and resistors, microstrip transmission lines which have superior signal transmission characteristics in high frequency region are generally used for transmitting signals among the elements. For increasing the integration density of the MMIC, the transmission lines are normally multilayered.
FIG. 1 shows a cross-sectional view of an MMIC according to the related art which has a multilayered wiring structure. As shown in FIG. 1, an active element 2 such as an HEMT and a passive element 3 such as a capacitor or a resistor are formed on the surface of a gallium arsenide (GaAs) substrate 1. The elements 2 and 3 are covered with a surface insulating layer 4, and a ground plate 5 is formed thereon which has a potential fixed to the ground potential. Insulating interlayers 6, 8, and 10 and wiring layers 7, 9, and 11 are alternately stacked on the ground plate 5, in accordance with the required level number of wiring layers.
FIG. 1 shows a three-layer wiring structure in which the wiring layers 7, 9, and 11 are alternatively stacked, with the insulating interlayers 6, 8, and 10 provided therebetween. Each of the wiring layers 7, 9, and 11 combines with the ground plate 5 to form each transmission line.
Integration density of the MMIC having the above multilayered wiring structure can be easily increased compared with an ordinary MMIC having a singlelayered wiring structure. However, power consumption correspondingly increases, and the generated heat results in deterioration in element characteristics and a decrease in reliability.
To suppress an increase in the temperature of the MMIC which is caused by the generated heat, a method is normally employed which includes the steps of thinly grinding the back side of a semiconductor substrate, and forming a metallic electrode thereon and bonding the metallic electrode to a heat-radiating plate so that the generated heat can escape from the back side of the semiconductor substrate. Semiconductor elements as heat sources are formed on the semiconductor substrate. Therefore, a high heat-conductive substrate must be used and the thickness of the substrate must be sufficiently reduced in order that the heat may escape from the back side of the substrate through its inside.
Nevertheless, the MMIC has a problem in that it is difficult to reduce the thickness of the substrate because there are many cases in which a compound semiconductor material such as gallium arsenide having a thermal conductivity lower than that of silicon is used as the material for the substrate and because damage caused by grinding the back side of the substrate may affect semiconductor elements on the surface of the substrate through its inside.
Although the heat generated by each semiconductor element radiates from the surface of the substrate through an insulating interlayer, it is also difficult to increase an efficiency of heat radiation from the surface of the substrate because an organic resin layer used as the insulating interlayer has a low thermal conductivity. In particular, the MMIC having the multilayered wiring structure has a problem in that it is further difficult to increase the efficiency of heat radiation since the insulating interlayer is multilayered to form an increased thickness.
It is an object of the present invention to provide a high frequency semiconductor device as an MMIC in which an efficiency of heat radiation is increased.
To this end, according to the present invention, the above object is achieved through provision of a high frequency semiconductor device including semiconductor elements provided on a semiconductor substrate, a surface insulating layer for covering the semiconductor elements, at least one wiring layer which is provided above the surface insulating layer, with at least one insulating interlayer provided therebetween, and which combines with the ground potential to form transmission line, and at least one heat-radiating stud which is provided in at least one throughhole so as to penetrate said insulating interlayers and so as not to penetrate said surface insulating layer.
Preferably, the at least one heat-radiating stud is provided on the surface region of the semiconductor substrate except for the area of the pads.
The heat-radiating studs may be provided in the throughholes.
The at least one heat-radiating stud may be connected to have a predetermined potential.
The at least one heat-radiating stud may be connected to have the ground potential.
A ground plate having the ground potential may be provided on the surface insulating layer, and the heat-radiating stud may be connected to the ground plate through the throughhole.
On the top external surface of the at least one insulating interlayer, the at least one heat-radiating stud may be connected to at least one pad supplied with a predetermined potential.
At least one heat-radiating plate for covering the at least one heat-radiating stud may be provided on the top external surface of the insulating interlayers.
The heat-radiating plates may cover the heat-radiating studs.
The heat-radiating plates may be provided for the heat-radiating studs so as to be mutually detached, with a distance provided therebetween.
On the top external surface of the insulating interlayers, the at least one heat-radiating plate may be connected to at least one pad supplied with a predetermined potential.
The at least one heat-radiating plate may be directly connected on the top external surface of the insulating interlayers so as to have an external potential.
The flip-chip connection may be used for connecting the external circuits.
A top protection layer may be formed on the heat-radiating studs and/or on the heat-radiating plates.
The high frequency semiconductor device may further include at least one wiring layer on the top external surface of the insulating interlayers. The protection layer may be formed so as to cover the wiring layer.
The at least one insulating interlayer may be made of an insulating resin material.
The insulating resin material may be one of polyimide and benzocyclobutene.
The wiring layers may be multilayered with the insulating interlayers provided therebetween.
The wiring layers may be singlelayered on the insulating interlayers.
According to the present invention, heat generated by a heat source such as a semiconductor element can be radiated from the surface of a semiconductor substrate through a heat-radiating stud. Thus, an increase in the temperature of the semiconductor elements can be reduced to be lower than that in the related art.
By providing a plurality of heat-radiating studs in an area except for the area of heat-radiating pads on a semiconductor substrate, limitations on the degree of freedom in a semiconductor-circuit layout can be minimized.
By fixing the potential of each heat-radiating stud to a predetermined potential, for example, to the ground potential, element characteristics can be stabilized. In order to fix the potential of the heat-radiating stud to the ground potential, for example, the heat-radiating stud may be connected to a ground plate provided on a surface protection layer, or may be connected to a pad supplied with the ground potential.
By providing a heat-radiating plate for covering the heat-radiating stud, the heat generated by the semiconductor elements can be efficiently escaped. Similarly to the heat-radiating stud, by fixing the potential of the heat-radiating plates to the ground potential, semiconducter element characteristics can be stabilized. For example, by connecting the heat-radiating plate to the pad which has a potential fixed to the ground potential, and establishing flip-chip connection of the pad, The generated heat is radiated from the heat-radiating plates to the exterior.
By providing a heat-radiating stud on a surface insulating layer, external moisture can be prevented from penetrating to the semiconductor elements through a throughhole. In addition, by forming a protection layer on the heat-radiating stud or the heat-radiating plate, adverse effects of the external atmosphere can be efficiently suppressed.
By using a low-dielectric organic resin as an insulating interlayer for separating a wiring layer, parasitic capacitance can be reduced, whereby the improvement of the semiconductor device characteristics and the superier heat radiating effects can be achieved. It is preferable in enhancing the advantages to use polyimide or benzocyclobutene as the organic resin.
Although the above structure can be applied to a single layer wiring structure, the advantages of the above structure are greatly enhanced when the above structure is applied to a high frequency semiconductor device having a multilayered wiring structure.
As described above, according to the present invention, an efficiency of heat radiation from a semiconductor chip is larger than that in the related art, because heat can be radiated from the surface of the semiconductor chip. Therefore, the present invention is advantageous in enhancing the performance of a high frequency semiconductor device which is high-integrated by employing a multilayered wiring structure and in increasing the reliability of the device.